The present disclosure relates to semiconductor structures, and particularly to electrical isolation structures for ultra-thin semiconductor-on-insulator (UTSOI) devices and methods of manufacturing the same.
Ultra-thin semiconductor-on-insulator (UTSOI) devices refer to semiconductor devices formed on an ultra-thin semiconductor-on-insulator (UTSOI) substrate. A UTSOI substrate can be employed to form various semiconductor devices that derive performance advantage through the reduced thickness of the top semiconductor layer and/or the reduced thickness of the buried insulator layer compared with normal semiconductor-on-insulator (SOI) substrate.
For example, the reduction in the thickness of the top semiconductor layer provides full depletion of the channel, thereby enhancing the electrical control of the channel by the gate electrode and reducing the leakage current in a field effect transistor. Further, the reduction in the thickness of the buried insulator layer can enhance control by a back gate electrode in back-gated field effect transistors.
While UTSOI devices, and especially UTSOI field effect transistors (FETs), are promising candidates for advanced high performance devices, several manufacturing issues need to be resolved before UTSOI devices can be manufactured with high yield. One such issue is erosion of shallow trench isolation structures that are employed to provide lateral electrical isolation between adjacent devices. Specifically, etch steps and/or cleaning steps are repeatedly employed to recess various material layers and/or to clean surfaces before further processing. Shallow trench isolation structures can be etched during such etch steps and/or cleaning steps. Further, underlying portions of the buried insulator layer can be eroded to a degree that a hole is formed underneath a cavity formed by removal of the shallow trench isolation structure and a top surface of a handle substrate is exposed underneath the hole. For example, silicon oxide-based shallow trench isolation structures are susceptible to HF-based etches that can be employed to preclean semiconductor surfaces before epitaxy or formation of a gate dielectric.
Such holes in the buried insulator layer can cause severe yield problems during formation of contacts to source and drain regions. For example, contact via holes can straddle over a sidewall around a hole such that a top surface of the handle substrate, which is typically a semiconductor substrate, is physically exposed at the bottom of the hole in addition to physically exposed surfaces of a source region or a drain region located in or above the top semiconductor layer. A direct electrical short can be formed between the bottom semiconductor layer and the source region or the drain region by a conductive material that is deposited in the hole to form a contact via structure.
Thus, a method of ensuring sufficient electrical isolation between a bottom semiconductor layer of a UTSOI substrate and electrical nodes in a top semiconductor layer of the UTSOI substrate despite the erosion of shallow trench isolation structures and portions of a buried insulator layer during processing steps is needed to provide functional and reliable UTSOI devices.